I would be extremely careful with putting anything on the lens. Some cameras have plastic, some have glass, some have a composite. You never know what something like RainX will do to the material. The cameras are designed to not have anything applied to them, so proceed with caution. Even some more abrasive cleaners and cloths can scratch a lens which won't show in daytime but can show at night in a dome for example because of IR refraction in that scratch if it is long and deep enough.